Effect of forced convection roasting on physicochemical and antioxidant properties of whole grain maize (Zea mays L.) and optimisation of roasting conditions

Bala, Shuaibu Mallam (2016-03)

Thesis (PhD Food Sc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Maize (Zea mays L.) is the most cultivated cereal and grain crop in the world and it is used as a staple food in developing countries such as Nigeria, South Africa, Mexico and economically less privileged countries. The grains of maize are processed into intermediate products (flour and meal) which are utilised for the production of different types of ready to eat foods. In most cases, the flour or meal used is refined (bran and germ removed) and not pregelatinised. Heat-processing methods of maize that uses dry heat reduced the nutritional quality of intermediate and end products. Forced convection roasting (FCR), a novel heat-processing technique, that has an additional advantage of using superheated steam was used to study the changes in physicochemical, proximate composition and antioxidant properties of Nigerian (S28, S33) and South African (H2G1, H7D1) maize varieties. Roasting temperature and rotating speed (determining roasting time) limits (150 to 220 °C) and (30 to 90 Hz), respectively were used for the roasting in a forced convection continuous tumble roaster (FCCTR). Roasting conditions (temperature/rotating speed) of the maize varieties were optimised for the production of whole grain flour or meal. Comparison of the proximate composition, antioxidant and pasting properties of the optimally processed whole grain flour or meal with raw whole grain flour or meal and an unroasted refined commercial maize meal (CMM) was made. The nutritional quality and antioxidant properties (content and activity) of the Nigerian maize S28 (yellow kernel) and S33 (white kernel) were not negatively affected by FCR. For the South African maize varieties, FCR did not show a negative effect on the proximate composition and antioxidant properties except the increase in total phenolics content of H2G1. Variations in the physicochemical properties such as bulk density, kernel hardness, colour and pH of the roasted maize varieties did not compromise the quality of the optimally processed whole grain flours. The prediction models of moisture content, whiteness index (WI), yellowness index (YI), total essential amino acids (TEAA) and total amino acids (TAA) had good fit (R2 >0.8) with the experimental data and non-significant (p ≤0.05) lack-of-fits. The desirability profiling of moisture content, WI and YI indicated 189.9 °C/90 Hz and 140.9 °C/49.8 Hz as the mean optimum roasting conditions of S28 and S33 maize varieties, respectively for the production of high quality whole grain flour or meal. Similarly, the desirability profiling of moisture content, WI, YI, TEAA and TAA showed the mean optimum roasting conditions of H2G1 and H7D1 white maize varieties to be 185.0 °C/65.5 Hz and 182.6 °C/55.0 Hz, respectively. The carbohydrate, crude protein, fat and fiber, ash, total phenolics and flavonoids as well as free radical scavenging capacity of the optimally processed whole grain flours did not significantly differ from those of the raw whole grain flours of each of the maize varieties. Both whole grain flours of the raw and roasted maize grains had higher proximate composition, total phenolics, total flavonoids and antioxidant activity than CMM, except the carbohydrate content which was found to significantly higher in the later. The optimally processed whole grain flour of each maize variety had a non-significantly lower pasting temperatures and significantly higher pasting viscosities compared to the raw whole grain flour. This indicated better pasting characteristics of the optimally processed whole grain flours with reference to the raw whole grain flours of the maize varieties. However, CMM had significantly lower pasting temperatures and higher pasting viscosities than the whole grain flours which indicated better pasting properties of the former. Considering the non-negative effect on proximate composition and antioxidant properties, and better pasting characteristics of whole grain flours of the roasted maize varieties, it could be concluded that FCR is a good alternative for roasting maize grains in the process of producing whole grain flours with the best quality for human consumption. It was also observed that the whole grain flours had better nutritional and antioxidant properties, but poorer pasting properties compared to CMM.

AFRIKAANSE OPSOMMING: Mielies (Zea mays L.) is die mees gekweekte graan in die wêreld en word gebruik as ‘n stapelvoedsel in ontwikkelende lande soos Nigerië, Suid-Afrika, Mexiko en ekonomies minder bevoorregte lande. Mieliepitte word geprosesseer vir die produksie van intermediêre produkte (meel en mieliemeel) vir die vervaardiging van verskillende tipes gereed-om-te-eet kosse. In die meeste gevalle is die meel of mieliemeel wat gebruik word, verfyn (semel en kiem is verwyder) en ook nie voorheen gegelatiniseer nie. Hitte-verwerkingsmetodes wat van droë hitte gebruik maak, verlaag die voedingswaarde van die mielie se intermediêre en finale produkte. Geforseerdekonveksie- verroostering (GKV), ‘n nuwe hitte-prosesseringstegniek met ‘n ekstra voordeel omdat dit gebruik maak van super-verhitte stoom, is gebruik om die veranderinge in die fisiochemiese eienskappe, voedingswaarde en antioksidant eienskappe van Nigeriese (S28, S33) en Suid- Afrikaanse (H2G1, H7D1) mieliebasters te bestudeer. Die volgende limiete is gebruik vir die verroostering: verroosteringstemperature (150 tot 220 °C) en verroosteringsspoed (30 to 90 Hz) soos gebruik gemaak tydens die geforseerde-konveksie-aaneenlopende-tuimelendeverroosteringsproses (GKATV). Verroostereingstoestande (verroosteringstemperature / rotasiespoed) van die mieliebasters is geoptimaliseer vir die produksie van heelgraan, sowel as meel of mieliemeel. Die voedingwaarde, antioksidant- en vergellingseienskappe van die geoptimaliseerde produksie van die heelgraanmeel of mieliemeel, saam met ongeroosterde heelgraanmeel of mieliemeel en ‘n verfynde kommersiële mieliemeel (KMM), is met mekaar vergelyk. Die voedingswaarde en antioksidanteienskappe (inhoud en aktiwiteit) van die Nigeriese mieliebasters S28 (geel mielie) en S33 (wit mielie) is nie negatief geaffekteer deur GKV nie. Die Suid-Afrikaanse mieliebasters wat blootgestel is aan GKV, het nie enige negatiewe effekte met betrekking tot die voedingswaarde en antioksidanteienskappe getoon nie, behalwe die toename in totale polifenolinhoud van H2G1. Variasie in die fisiochemise eienskappe soos bulk-digtheid, pit hardheid, kleur en pH van die geroosterde mieliebasters het nie die kwaliteit van die optimaalgeprosesseerde heelgraanmele beïnvloed nie. Die voorspellingsmodelle van die voginhoud, witheidsindeks (WI), geelheidsindeks (GI), totale essensiële aminosure (TEA) en totale aminosure (TA) het goeie verband getoon (R2 >0.8) met die eksperimentele data wat nie-beduidende (p ≤0.05) gepas het. Die gewenste profiel van die voginhoud, WI en GI het daarop gedui dat 189.9 °C/90 Hz en 140.9 °C/49.8 Hz die gemiddelde optimale verroosteringskondisies vir S28 en S33 mieliebasters, onderskeidelik, was vir die produksie van hoë kwaliteit heelgraanmeel of mieliemeel. Soortgelyk het die gewenste profiel vir die voginhoud, WI, GI, TEA en TA daarop gewys dat die gemiddelde optimale verroosteringskondisies vir H2G1 en H7DI witmieliebasters onderskeidelik 185.0 °C/65.5 Hz en 182.6 °C/55.0 Hz was. Die koolhidraat-, ru-protein-, vet en vessel-, as-, totale fenole- en flavanoïde inhoud, asook die vry-radikaal-aas-kapasiteit van die optimaal-geprosesseerde heelgraanmele het nie-beduidend van die rou heelgraanmele verskil, vir elk van die mieliebasters. Beide heelgraanmele van die rou en geroosterde mielies het ‘n hoër ru-protein-, vet en vesel, as-, totale fenole- en flavanoïde inhoud, asook vry-radikaal-aas-kapasiteit getoon as die KMM, behalwe die koolhidraat inhoud wat beduidend hoër was in die KMM. Die optimaal-geprosesseerde heelgraanmele van elk van die mieliebasters het ‘n nie-beduidend laer vergellingstemperatuur en ‘n hoër vergellingsviskositeit getoon in vergelyking met die rou heelgraanmeel. Dit het gedui op beter vergellingseienskappe vir die optimaal-geprosesseerde heelgraanmele met verwysing na die rou heelgraanmele van die mieliebasters. Nieteenstaande het die KMM beduidend laer vergellingstemperature en hoër vergellingsviskositeit getoon as dié van die heelgraanmele wat daarop gedui het dat die vergellingseienskappe van laasgenoemde beter was. As die koolhidraat-, ru-protein-, vet en vessel-, as-, totale fenole- en flavanoïde inhoud en die vry-radikaal-aas-kapasiteit, asook die vergellingseienskappe van die heelgraanmele van die mieliebasters in ag geneem word, kan daar tot die gevolgtrekking gekom word dat GKV ‘n beter alternatief vir verroostering van mielies is vir die produksie van heelgraanmele om die beste kwaleiteit vir menslike gebruik. Dit is ook opgemerk dat die heelgraanmele beter voedingswaarde en antioksidant eienskappe gehad het, alhoewel swakker vergellingseienskappe in vergelyking met KMM.

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